1. Field of Invention
The present invention relates to papermaking fabrics woven from synthetic materials to be used primarily in the wet end, or forming section of a papermaking machine, although use of these fabrics may also extend to other applications.
In the usual flat bed type papermaking machine the wet end, or forming section is known as a Fourdrinier. Paper pulp is deposited from a headbox upon a large foraminous screen in the form of an endless belt that is propelled over and around machine rolls, dewatering devices, suction boxes and other machine elements. The pulp is carried away from the headbox and water drains through the belt to set up the initial paper web, and when the web arrives at the foot end of the Fourdrinier it releases from the belt, or fabric, to move successively through press and dryer sections of the papermaking machine to produce a dried paper sheet. The fabric of the present invention is primarily intended for use as such a screen in the Fourdrinier and other papermaking machines such as twin wire machines.
2. Description of Prior Art
Paperforming fabrics are commonly constructed of monofilament synthetic threads, and polyester is presently the prevalent material for these fabrics. The fabrics are woven in a variety of weave patterns, and in such mesh counts and thread diameters as to suit the particular machines and paper grade for which they are intended. Typical weave patterns include the semi-twill weave, in which threads of one thread system pass across two threads of the other thread system, then interlace through the fabric and pass across a single thread of the other system on the opposite side of the fabric. This weave may also be termed a three-harness weave in reference to the arrangement of the harnesses in the weaving loom. A more common weave configuration is the four-harness in which the threads of one system pass across three of the threads of the other system and then are interlaced to the opposite side of the fabric to pass across a single thread of the other thread system. Four-harness configurations can be arranged in a twill pattern or a broken satin pattern, or four-harness weaves can also be used in a full-twill pattern in which the threads of one system will pass across two threads of the other system on one side of the fabric and then interlace through the fabric to pass across two threads of the other system on the other side of the fabric. Five or more harnesses can also be employed for paperforming fabrics.
Synthetic threads, and fabrics from which they are made, have a tendency to stretch, or elongate when on a papermaking machine and subjected to the tension loads arising from driving the fabrics over and around the Fourdrinier machine elements. To overcome stretching steps are taken to minimize the crimp in the machine direction threads. One technique is to highly stretch the fabric in a thermosetting process subsequent to weaving that is a necessary part of the manufacturing process. This increases the fabric modulus, or resistance to stretch, so that subsequent elongation on the paper machine will be reduced. A second technique is to employ a weave pattern in which the machine direction threads do not interlace between the cross machine direction threads as frequently, and for this purpose it has been popular to resort to the four-harness weaves. The resultant reduction in transverse interlacing of the machine direction threads reduces the total crimp of the threads, so that elongation on the paper machine will be reduced. Resort to five-harness patterns is an extension of this practice to reduce total crimp, and at the same time, in both the four and five-harness weaves there is an adequate number of interlacings of the threads to form sufficient vertical crimp to render the fabric stable. By stable is meant the maintenance of individual threads in their position as woven, so that shoving and other thread displacement will not take place during the paper-making process. Adequate stability can also be achieved in these fabrics for holding seams in flat woven fabrics.
However, there is a tendency of threads to "twin" in the manufacture of monofilament synthetic fabrics in certain four and five-harness weaves. "Twinning" is a phenomenon in which threads extending in the same direction tend to pair-up with one another, so that as one views the fabric it becomes evident each thread is more closely spaced to a thread on one side than on the other. Twinning results in nonuniform spacing between threads and if this unevenness becomes excessive the pulp fibers arrange themselves on the fabric during the water draining process in a manner that wire mark on the finished paper becomes objectionable. This wire mark may be manifested by visible lines running across the finished paper where the space or gap between fabric threads is excessive, and this unsightly result is usually more noticeable in the weft thread direction than the warp thread direction. For paperforming fabrics that are woven flat and seamed these weft induced markings run in the cross machine direction.
The amount of twinning is determined by first measuring the distance between the wider spread threads at points along their lengths which are medial the threads of the other system, and then measuring the distance between the threads of a pair, and taking the ratio of these two measurements. Twinning may run as high as a ratio of 2.0 in the weft thread direction of flat woven fabrics, and at values in this vicinity and above paper marking usually becomes objectionable. The present invention is directed toward the reduction of twinning as one of its several objectives.
The invention is also directed to facilitating the seaming operation in the manufacture of flat woven synthetic fabrics. In flat weaving the fabric is woven as a long flat piece of goods, and after removal from the loom the two ends are brought together and joined by a seam to form an endless belt. Seams for synthetic fabrics are usually formed by removing a plurality of weft threads at each end of the fabric, then interdigiting the exposed warp threads and weaving additional weft threads into them with the same weave pattern as the rest of the fabric. This weaving of a seam is a manual process requiring a high degree of skill and dexterity, and is extremely time consuming. The warp ends must be kept straight and separated from one another during the seaming process, and curl, twisting or entanglement of the warps with one another frustrates efficient manufacture.
The present invention aids in minimizing entanglement of warp threads, so that seaming can be carried out more efficiently. To achieve this goal, the invention employs warp threads that have generally rectangular cross section configurations. It has been found that such a configuration will improve the ability of warp ends to lie straight in the direction of the fabric without curling and twisting.